About half the energy used in a modern supermarket is consumed by backroom refrigeration equipment, primarily compressors, condensers and related components. This equipment must be properly sized to provide enough refrigeration capacity to maintain the qualify of food in the refrigerated cases.
The difficulty in determining the proper size or capacity of a refrigeration system lies in the fact that the system cooling load changes dramatically depending on a number of unrelated factors: time of day, outside temperature and humidity, inside temperature and humidity, the manner in which the cases are stocked, the frequency and duration of use by customers, and so forth.
To account for this constantly changing load, refrigeration compressor systems currently used for supermarket product refrigeration have to be designed with enough capacity to function properly under the worst possible conditions--the hottest, most humid days of the year. Since worst case conditions occur on the average only about two percent of the time--six or seven days of the year--the prior art systems are inefficient about 98 percent of the time.
Evolution of Compressor System Design
There are three primary types of prior art refrigeration compressor systems: conventional (single compressor) systems, parallel (multiple equal compressor) systems, and dissimilar (multiple unequal compressor) systems.
Conventional Systems. The first compressor system widely used in supermarkets was the conventional, single compressor, system, in which a single compressor system is used for each "application" (i.e., case or set of connected cases with similar types of product therein) in the store. In these conventional systems, capacity control is very simple--the system is either turned on or off. This is acceptable with small compressors, but for larger compressors it is seldom satisfactory because of the fluctuations in controlled temperature.
Under light load conditions the conventional systems can suffer damage from compressor short cycling (i.e., turning on and off too frequently). Many stores reduce the low pressure cutout setting on these system to a point below the design limits of the system in order to prevent short cycling. As a result, the compressor may operate for long periods at extremely low evaporating temperatures. Operating the system at temperatures below those for which it was designed leads to overheating the motor compressor and to inadequate oil return to the compressor. These conditions can cause compressor damage and failure.
Other major disadvantages of conventional compressor systems are as follows. Because of their cyclic capacity control, conventional systems cannot maintain case temperature temperatures. Typically, the variance is 8.degree. F.
Conventional systems cycle on and off frequently when the condensing temperature is low because the capacity of the system becomes very large compared to the load, and therefore conventional system are unable to take advantage of the low condensing temperatures at which operation would be the most efficient. The present invention can take advantage of low condensing temperatures--and thus be more efficient--because the system can reduce its capacity so that the system's capacity more closely matches the load.
Conventional system use semi-hermetic compressors which have high failure rates. Repairing a semi-hermetic compressor requires removing the unit from the store and returning it to the manufacturer or to a rebuilder.
Parallel Systems. The next major step in the evolution of refrigeration system design involved systems with two, three and four equal-sized compressors configured for either low or medium temperature applications. Whereas a typical store might require 18 to 25 conventional compressor units, it would require only six to eight two-compressor parallel systems, and only two four-compressor systems.
Parallel systems offered a modest increase in capacity control--three to five steps as compared to the two steps in conventional systems. Also, failure of any one of the compressors does not result in direct product loss unless the system is operating near worst case loading. Other advantages of these systems was that these systems can use compressor hot-gas defrost in place of the electric heat defrost used in convention systems, heat reclaiming is more cost efficient than in conventional systems, and parallel systems occupy less space than convention systems, making it possible to have smaller machine rooms.
Major disadvantages of the parallel systems include lower efficiency (due to the need to operate with the lowest common suction pressure in the joint suction manifold), oil distribution problems (caused by different compressor oil pumping rates, interconnected compressor crankcases and uneven oil return), higher installation and service costs caused by system complexity, and higher (typically five times higher) costs for replacing refrigerant lost via leakage and contamination.
Dissimilar Systems. In an attempt to improve the relatively poor energy efficiency of parallel systems, dissimilar systems with three or four binarily weighted compressors (i.e., with nominal capacity ratios of 1:2:4:8). The most common dissimilar systems have three compressors with eight capacity steps, as compared to five steps for a four compressor parallel system. A typical store that would require 18 to 25 conventional compressor systems and two four unit parallel systems (with a total of eight compressors) would typically be configures with five three unit dissimilar systems (with a total of 15 compressors).
Dissimilar systems have two primary advantages over parallel systems: the additional capacity steps permit better matching of compressor capacity to case heat load; and energy efficiency is better because fewer application pressures are multiplexed into a common suction pressure.
Compared to conventional systems, dissimilar systems offer some of the same advantages of parallel systems, namely hot gas defrost and somewhat lower heat reclaim costs.
However, to effectively use the extra capacity steps requires the addition of sophisticated, expensive controls. Also, like parallel systems, dissimilar systems: require complex oil distribution systems, have lower energy efficient ratios than conventional systems because of the multiplexing of suction pressures, and have higher installation and servicing costs than conventional systems due to the system's increased complexity. Also, even the best dissimilar system still suffer case temperature swings of 4.degree. F.
In summary, despite their inherent weaknesses, conventional single compressor systems remain the most commonly used compressors in supermarkets largely because (1) conventional systems are dedicated to single applications, which makes it possible to more closely match the compressor size to its load than for other types of systems, (2) operating at a single suction pressure results in a higher energy efficiency ratio, and (3) conventional systems are less complex than parallel and dissimilar systems, and hence, easier to install and maintain.
How the Present Invention Differs from Prior Systems
The present invention is a compressor system with continuously variable capacity. This is achieved by using a direct drive motor with a wide range of operating speeds to drive a standard reciprocating compressor. A control system continually tracks the temperature in the application, and the pressure in the suction line, and determines the best motor speed to match the current load on the system. Since the motor speed is continuously variable, the system can adjust its heat load capacity to closely match the current load on the system.
The present invention has the primary characteristics of the ideal refrigeration capacity control system. First, it continuously adjusts to load. Second, full load efficiency is unaffected by the capacity control mechanism. Third, there is no loss of efficiency at partial loads. Fourth, there is no reduction in the reliability of the compressor caused by the capacity adjustment mechanism.
Since refrigeration compressors made in accordance with the present invention can reduce capacity to match reduced loads, these compressors are cycled off and on much less frequently than prior art compressors. By substantially reducing the frequency of stressful compressor restarts, and by virtually eliminating compressor slugging (i.e., drawing too much refrigerant when turning on), the present invention reduces maintenance costs.
Another important advantage of the present invention is that it can maintain case temperature within 1.degree. F. of a specified setpoint. This compares to 8.degree. F. swings for conventional systems, and 4.degree. to 6.degree. F. for dissimilar systems, thus holding out the promise of improved product quality and longer shelf life.
It should be noted that the present invention uses reciprocating (e.g., 2 or 4 piston) compressors, which are required in medium temperature (below 55.degree. F.) and low temperature refrigeration (below 20.degree. F.) systems. In the commercial refrigeration industry practically all of the compressors used are semi-hermetic compressors (i.e., reciprocating compressors with a motor mounted on the same drive shaft as the compressor, built together in a semi-hermetic housing).
Until the present invention, it has been generally assumed by the refrigeration industry that open direct drive compressors were too expensive and unreliable for commercial refrigeration. One of bases for this assumption has been that, in the prior art systems, alignment of the motor and compressor in open direct drive compressors was a difficult and expensive process. Minor misalignments caused seals to deteriorate, ultimately resulting in vibration and mechanical failure. The present invention solves this problem with a new bell housing that couples a motor to a compressor and ensures proper alignment.
It may also be noted that continuously varying the capacity of a prior art semi-hermetic compressor is generally not practical. The speed of semi-hermetic compressors cannot be varied significantly because, at speeds below the compressor's normal speed (e.g., 1750 rpms), the compressor will receive insufficient refrigerant mass flow to prevent motor burn out. Also, these compressors generally use forced gear oil pumps which are designed to provide sufficient lubrication only when the motor runs at a specified speed.